Method and system for estimating the efficiency of the lungs of a patient

The present disclosure relates to methods and systems for estimating an efficiency of lungs of a patient receiving respiratory care. A blender has a primary input port for receiving a first gas to be delivered to the patient and one or more secondary input ports for receiving a second gas to be delivered to the patient from one or more gas sources. A patient-side port of the blender delivers the first and second gases to the patient. A gas composition sensor measures a fraction of the first gas and a gas flow sensor measures a flow of the first gas. A controller causes a sequential delivery of the first and second gases to the patient and estimates a functional residual capacity of the patient based on measurements from the gas composition sensor and from the gas flow sensor. The controller may also estimate a cardiac output of the patient.